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Monday, May 6, 2013

Europa Clipper Update

Planetary geologist Philip Horzempa returns with a new post giving an update on the proposed Europa Clipper mission.

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President Obama’s recently released FY2014 budget
proposal, unfortunately, contains no funding for a mission to Europa. In fact, the budget document states that NASA
not only is not funding such a mission, but that it cannot fund it. Several sources of budget constraints appear
to be stalling any new start. In addition
to the sequester, NASA’s Science Mission Directorate has the ongoing money
drain of the Jams Webb Space Telescope (JWST).
That funding burden will not lessen until about 2017 – 2018. One could imagine that NASA may see a funding
wedge appear at about that time, with a new start for a Europa mission possible
in FY 2015 or 2016. The early years of a
space project require minimal funding, allowing a program to begin Phase A and
Phase B (design and definition) a few years before the fiscal “heavy-lifting”
of Phases C and D (detailed design, construction and testing) .In the meantime,
the Europa team has continued to refine the design of what they refer to as the
Europa Clipper (see Van’s post of September 24, 2012). Whenever they are given the “go-ahead” from
the White House, they will have a mission ready to proceed to implementation.

The Europa science community believe they have developed
a cost-effective, yet scientifically compelling, mission to the ice-covered
Galilean satellite. After considering an
orbiter, the consensus is that a multi-flyby spacecraft would return more
science for the same cost ceiling. The
Europa Clipper embodies the modified FBC (faster, better, cheaper)
approach. It is seeking to capture as
much of the Jupiter Europa Orbiter (JEO) flagship science as possible using a
smart, elegant, lower-cost design. This
past January, the Europa team presented the results of their latest “scrub” of
the Clipper mission. This Europa Clipper
design refinement can be seen here.

The plan is to launch in 2021, followed by one Venus and
two Earth gravity assists. Six years
after launch, with the gravity assist of a Ganymede flyby, the Clipper will
enter orbit around Jupiter. Over the
next 2.5 years, it will perform 32 flybys during its prime mission, with
closest approach altitudes of 25 - 100 kilometers (actually 34 total flybys
will occur, but only 32 are optimal for science). In order to reduce planning costs, the
timeline of each flyby will be essentially identical. (Figure 1)
However, the trajectory of each flyby will bring it over a different sector of
Europa. This will provide global
medium-resolution coverage from the Topographic Imager.

Figure 1. Flyby timeline. Click on image for a larger version.

It was felt that the Europa Clipper mission should also
provide data that would feed-forward to a future soft lander. This concept of reconnaissance has seen a
rebirth at NASA, with ongoing orbital missions at the Moon and Mars. The addition of a Reconnaissance Camera was
deemed to be essential for providing images for landing site surveys (
lander-scale characterization of the surface is needed). The Recon camera (a push-broom design) will
produce 20 x n km images at resolutions of as fine as 0.5 meters. The limitation on the number of such
high-resolution images comes from the large amount of data in each photo. In
turn, the swath length will be determined by the amount of down-link
time available. The Recon camera will
utilize an innovative flip-mirror to enable stereo imaging of a scene in a
single pass. It will be able to obtain
views 15 degrees from nadir (Figure 2)
It is believed that about 15 candidate landing sites will need to be
surveyed in order to be able to down-select to 2, a primary and a backup. That selection will be done by some future
team of Europa Lander scientists and engineers.

Figure 2. High resolution camera flip mirror to allow stereo imaging.

A separate, smaller, and gimbaled gravity science antenna will allow the
collection of gravity data during flybys. (Figure 3). Because the cameras and other remote sensing instruments are mounted to the spacecraft body, the main antenna cannot be pointed to Earth during flybys to allow tracking for gravity measurements. The separate antenna will be kept pointed at Earth during flybys to permit the important gravity measurements that will reveal much of the internal structure of Europa.

Figure 3. Gravity science antenna.

During this latest
iteration, the Europa team was allowed to raise the cost cap from $1.7 billion
to a total of $2.0 billion. (This is
still less than half the estimated cost of the previously proposed Jupiter
Europa Orbiter.) This increase allowed
the addition of a Magnetometer and Langmuir Probes to the payload suite. Rounding out the instrument complement are an
Ice-Penetrating Radar, a Thermal Imager, a Neutral-Mass Spectrometer and a
Short-Wave Infra-Red Spectrometer. Figure 4 shows some of the payload
complement and where they will sit on the spacecraft.

Figure 4. Europa Clipper instruments.

The highly-capable instrument suite is one reason that
the Europa Clipper would cost more than missions such as JUICE or the proposed
Io Observer. The scope and resilience of
the Clipper mission means that it must survive an intense radiation exposure
over its 2.5-year mission. This
data-intensive mission must also use a reliable, high-energy power source.

The Europa Clipper spacecraft benefits from the heritage
of the Galileo and Juno Jupiter Orbiters in its approach to radiation
protection.The Clipper will utilize 150 kg. of dedicated radiation shielding
which is one-half of that planned for the earlier JEO (Jupiter Europa
Orbiter) proposal. The Clipper will use a scheme of nested
radiation protection for its electronics (Figure 5).

For example, the Spacecraft structure and propulsion
system will provide a measure of radiation protection, essentially for
free. With intelligent placement, the
project will utilize much less expensive 100 and 300 kilo-rad hard parts. Individual payload electronics have their own
shielding, while the use of a central electronics vault is also part of the
protection plan. As a result of this
approach, the Clipper team will not need to fund an expensive development
effort to build mega-rad hard avionics.

The Europa Clipper mission will be data-intensive. In order to downlink this data efficiently
and cheaply, the Clipper will use mass-memory-storage. The spacecraft will
leisurely downlink the data from each close encounter with Europa during the two weeks between flybys. This will avoid
the more costly, and power-hungry, approach of near-real-time broadcast during
flyby.

Over the course of its prime mission, the Clipper will
return a Terabit of data, including high-resolution images, radar soundings,
magnetic field measurements, compostion spectra, and gravity science. In order to return all of this date, a robust
energy source is required. There are
three energy supply options, two of which are thermal-electric and one solar.

Solar panels would be the lowest cost, highest mass
option. However, they pose the risk of
not providing enough power over the lifetime of the mission. The Europa Clipper's orbit has a low
inclination causing it to pass through the most intense radiation environment
in the solar system.This would cause aggressive degradation of solar cells,
such that their power output would be increasingly compromised as the mission
progressed. The Juno orbiter is able to
use solar power because its high-inclination polar trajectory enables it to
avoid most of the high radiation zones that are concentrated over Jupiter's
equator. This is true even though it
flies much closer to the gas giant than the Clipper ever will. ESA’s JUICE spacecraft is able to use solar
energy mainly because it only flies near Europa twice during its mission.

The proposed Io
Observer would also use solar panels. It
avoids high doses of radiation by orbiting Jupiter in an inclined orbit. Europa Clipper is unable to utilize such a
high-inclination orbit because that would result in flyby velocities too great
to allow its Infra-Red and Ice-Penetrating Radar to gather useful data.

This leaves the two thermal-electric options. These power systems utilize the heat
generated by the decay of Plutonium-238 to drive thermal-electric power conversion
units. One of these, the Advanced
Stirling Radioisotope Generator (ASRG) design is actually still in development,
although at a high level of maturity.
NASA chose not to pursue a Discovery mission that would have utilized
one of these units. In light of that
decision, the agency will still take the two ASRG development units to flight
status this year. They will then be
placed in storage, awaiting a mission.
If this power source is chosen, then the Clipper would utilize four ASRG
units.

However, before the ASRG design would be approved for
the Europa Clipper, more work would need
to be done. The radiation hardness of
the Generation-1 ASRG units is not sufficient for the Europa mission and there
are also lifetime demonstration issues.

The other thermal-electric option is the Multi-Mission
Radioisotope Thermal-electric Generator design (MMRTG). This system is the 1st
new radioisotope power system developed in over 20 years.It has advanced to
actual flight status, with the first MMRTG flight unit, F-1, now sitting on the
surface of Mars, powering the MSL rover.
Its backup, F-2, is in bonded storage at the Rocketdyne plant in Canoga
Park. It has been operated and has shown
good performance. It is now slated to
fly onboard the 2020 Mars Rover, i.e., MSL-2.

The next unit,
F-3, is the flight spare for Mars 2020.It is now under construction, with
completion set for this month. If not
needed for the Mars 2020 rover, then F-3 would be available for a mission to
Europa.In addition to F-3, three more MMRTG units would be needed for the
Clipper. There are plans for infusing
new technologies in the next generation of MMRTG.These would produce 150, or
even 180 watts, as compared to 120 watts for the 1st
generation.

There are a
number of issues that need to be considered if one of the thermal-electric
options is chosen. ASRG development
seems to have begun during the short-lived Prometheus program. An engineering unit at NASA Glenn has
accumulated over 10,000 hours (14 months) of operation so far. The maturity level for the ASRG units is
high, but they are more expensive than an MMRTG and have yet to fly in
space. On the other hand, their power
conversion efficiency of 30% means that they are more frugal than MMRTG units
(9% efficiency) with the Plutonium supply.

The MMRTG design has several advantages over the
ASRG. First, as noted, an MMRTG is now
in space. The design has high
reliability and low cost. In addition,
the ASRG utilizes kinetic energy as one stage in it power conversion. It is still to be determined whether the
resulting vibrations would make it incompatible with a Europa mission. If so, then the vibration-free MMRTG would be
at an advantage. In addition, the
re-start of Plutonium production in the U.S. may make the use of an MMRTG for
Europa more plausible. One factor that
had favored the use of ASRG units for space missions was the shrinking
inventory of Pu-238 in this country.
However, if the goal of producing 1.5 – 2.0 kg of Pu-238 per year is
met, then that concern will be eased.America now has about 10 kg of older,
aging Pu-238. The new Pu-238 can be
blended with the old material producing the desired power density.

Over the next 18 months the Europa project team will be
conducting a comprehensive trade study, comparing all viable energy
options. The variables to be considered
include cost, risk, robustness, design compatibility, and implementation
feasibility. This effort will go a long
way towards choosing the most appropriate system for the Clipper.

The Clipper team is very interested in the idea of
hosting several nanosats that would be deployed in the vicinity of Europa. This is contingent upon the use of the Space
Launch System (SLS) heavy-lifter. Only
that rocket would provide the needed mass margin required if the Clipper is to
carry small satellite payload elements.
However, if pursued, the working concept for the Clipper could provide
the necessary housekeeping, deployment and radio-relay capabilities. In addition, thought is being given to
utilizing an intermediate orbit insertion module that would allow several
nanosats to enter orbit around Europa.

If these nanosats can be accommodated, then the Europa
team would like to cooperate with the growing American small-sat
community. There is a desire to get
feedback from engineers and scientists on the best way to use these
probes. There are a variety of options
that could use a single smallsat, or a network, with instruments such as
magnetometers or cameras. These probes
could be orbiters, “Ranger-style” crash landers, or even hard landers that
might operate for a short time after impact.
Resource and cost constraints will be tight, but if these
mini-probes could fit, then the Europa
team is interested.

Still to be decided this year is how, or if, a total of
$75 million of new funding is to be spent.
In this year’s budget, Congress specifically earmarked that sum for
development of a Europa mission. There
have been rumors that NASA’s operating plan for this year’s budget, due to be
delivered to Congress soon, will seek to spend that money on other agency
projects. In response to such concerns,
Senators Diane Feinstein and Barbara Boxer joined with Congressmen Adam Schiff and John Culberson in
sending a letter to NASA. They point out to NASA that
funding levels for its science programs “will remain consistent with the
structure directed by Congress.”
Essentially, they are reminding the agency that the Constitution gives
the power to say how the nation’s money is spent to the Congress. The Executive branch has limited leeway in
how it interprets Congress’ appropriations legislation.

How this will turn out is difficult to gauge. This is not the first time such a struggle
has occurred. For years, the Congress
earmarked funds for development of a Solar Probe mission. Eventually, NASA got the message and awarded
a new start for the Solar Probe Plus spacecraft. About 10 years ago, when NASA was trying to
eliminate funding for the New Horizons Pluto probe, Congress specifically
earmarked funding for that mission, enabling it to proceed.More recently, after
the Obama Administration canceled the Ares 5 heavy lift rocket in its FY 2011
budget proposal, the Congress (especially the Senate) was not pleased. They directed NASA to pursue an alternate heavy
lifter, the SLS (Space Launch System), which is essentially a scaled-back
version of the Ares 5.That launcher is now on track for its first mission in
2017.

If NASA does agree to spend the $75 million (more like
$70 million after sequestration) this year for Europa mission preparation,
there are several ways that the money could be usefully spent. Instrument development, launch vehicle
requirements and power system options could be funded, as well as studies to
define the loads on the Clipper during launch.
Much will also depend on whether Congress again earmarks funds for a
Europa mission in the new FY 2014 budget.
If it does, then the tug-of-war with the Administration will continue
with the future of Europa exploration hanging in the balance.

Editorial Note from Van: If you are an American citizen and you would like to see NASA continue work on the Europa Clipper, remember to let your Congressional representatives know. Visit the Planetary Society's website for instructions on how to do so. You can also follow the latest information on the budget on Twitter at #fundPlanetary.

12 comments:

What about a VASIMR subsidy of $200M to the first mission for each major planetoid system? It would be a good space platform for the GOP. What helps one mission helps the ice moon microbe searches. Ev en if we don't find much more, we might be able to improve the Drake Equation solution.

VASIMR, being able to go fast or slow and alter the speed, on the same platform...it seems simple enough to work and with radiator costs should be economical. Risky robot missions are okay. VASIMR can't get humans to the nearest star, so shouldn't anger ET.

This is an exciting mission that appears to be maturing in time to begin a new start in time for the 2021 launch.It is important to continue pressuring congress to impart their will on the administration to enable this mission by that launch date.I remember when Pluto Express was canceled prompting the Planetary Society to become involved with a letter writing campaign. The result was the New Horizons mission. It too was funded at the objection of the administration at the time.

I want to correct an error that I made in the article. The actual time between flybys for the Europa Clipper will be more like 2 weeks, rather than the 4 weeks which I indicated in the article. Please excuse the mistake.

Promising progress is being made it seems, good news, good article! Such a shame that the current priorities are all dictated by politics and pork to congressional districts. Imagine if the Planetary Science community had the final say in what missions get flown and how much they should cost. Scientists decided on what science should get funded! Gosh, I get chills up my spine just thinking about it! Maybe one day in an enlightened future age.

Scientists DO decide what science they want to see funded--see the Decadal Survey link. It's my understanding this mission has not yet made the cut, perhaps partly because of the MMRTG/plutonium-shortage issue, partly because of the great cost of the previous iteration of this design, etc.

Mentioning using SLS in the context of a planetary science mission is laughable--that would drive up the LAUNCH price by at least a billion, much less the development cost of all the additional science payloads. Not to mention the increased risk of tying your mission to an as-yet-undeveloped launch platform.

The fact that all the members of Congress upset about the $75 million represent districts where that money would be spent (JPL) makes this look much more like pork. Not that it doesn't have SOME merit--most pork does in fact have local merit. It's just not what would be chosen, considering the entire American interest (for example, other very good planetary exploration projects).

Scientists get the _final_ say on funding, when they provide all the funds. Since the entire American people provide the funds, the entire American people get a say in how they are spent. That is how it should be.

IMO, SLS is a bigger drain on NASA resources than JWST. When i was a kid, i thought that the USSR could use a couple Proton launches and beat us to the Moon. Maybe more than two would have been needed. These days, i think a bunch of Falcon 9 launches would get us to Mars.

Assuming that NASA doesn't get a budget, perhaps ESA could pick up the mission. And after all. Any life found on Europa would be Europeans.

About Me

You can contact me at futureplanets1@gmail.com with any questions or comments.
I have followed planetary exploration since I opened my newspaper in 1976 and saw the first photo from the surface of Mars. The challenges of conceiving and designing planetary missions has always fascinated me. I don't have any formal tie to NASA or planetary exploration (although I use data from NASA's Earth science missions in my professional work as an ecologist).
Corrections and additions always welcome.